Image Dynamic Range and Color Gamut
As used herein, the term dynamic range may relate to the range of luminance (e.g., intensity) in an image. The dynamic range in real-world scenes is large. Different image and video applications in use today for the capture, representation, and presentation of image and video signals may have different dynamic ranges. For example, photographic negatives can have a relatively large dynamic range, while photographic prints, some currently existing (e.g., conventional) television (TV) sets and computer monitors may have a smaller dynamic range.
As used herein, the term “standard dynamic range” (SDR) (sometimes referred to as “low dynamic range” or “LDR”) may relate to the dynamic range associated with many current TVs or monitor displays. In contrast to SDR images, high dynamic range (HDR) images contain essentially all of the dynamic range in an original scene on which the HDR image is based. SDR images are typified by the images rendered with the 8-bit displays in widespread use today. HDR images may be rendered with 10-14 bit displays.
For a number of applications such as compression for distribution and display, encoding HDR may not be necessary and may in fact be somewhat computationally expensive and/or bandwidth consumptive. However, for some of these modern applications, SDR images may simply not suffice for all purposes. Instead, such applications may more advantageously use, create, store, transmit or render images that may be characterized by a Visual Dynamic Range (VDR). VDR images encompass essentially all of the luminance and color that a typical human visual system (HVS) can simultaneously perceive (e.g., visually perceive at any given time).
The entire range of human-visible luminance essentially spans the HDR range. In contrast, the range of luminance that is simultaneously visible comprises the VDR range, which while somewhat narrower than the entire HDR range, still covers a significant span. In further contrast to the HDR and VDR however, the range of a typical 8-bit gamma-mapped displays spans the SDR, range.
As used herein, the term color gamut may relate to a certain subset of colors; e.g., in relation to human visual perception and/or the color reproduction capability of a given display device. Color gamuts are commonly represented as areas within the CIE 1931 chromaticity diagram, which is familiar to artisans skilled in fields related to color science and arts, images, video, displays, cinematography and photography (e.g., “color, image and video”). Artisans skilled in these fields realize that a more-or-less “horseshoe” shaped region of the CIE 1931 chromaticity diagram represents is the entire range of possible chromaticities.
For any display device however, there are physical limits to the set of colors that can be reproduced or displayed. For example, existing display devices such as a cathode ray tube (CRT), liquid crystal display (LCD), or plasma display typically cover only a relatively small area in the entire color space. With reference to the CIE 1931 chromaticity diagram, artisans skilled in the color, image and video fields recognize that the color gamut available in most conventional devices may be represented as a triangle shape, within the entire, significantly (e.g., substantially) larger horseshoe shape area of the CIE 1931 chromaticity diagram, which represents an entire range of possible chromaticities that may exist. Certain colors that cannot be displayed, rendered or reproduced within a particular color model may be referred to herein as out of gamut. Additional processing is typically needed to handle, reproduce and display these colors. Such additional processing may be referred herein to as gamut mapping. As used herein, the term wide color gamut (WCG) may relate to a color gamut that encompasses more color than the devices delimited to the smaller triangle within the horseshoe shape of the CIE 1931 chromaticity diagram and, essentially at a limit, all the colors that the HVS is capable of perceiving.
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section. Similarly, issues identified with respect to one or more approaches should not assume to have been recognized in any prior art on the basis of this section, unless otherwise indicated.